Microwaves are electromagnetic energy waves with very short wavelengths, typically ranging from a millimeter to 30 centimeters peak to peak. In high-speed communications systems, microwaves are used as carrier signals for sending information from point A to point B. Information carried by microwaves is transmitted, received and processed by microwave circuits.
Packaging of RF and microwave microcircuits has traditionally been very expensive. The packaging requirements are extremely demanding--very high electrical isolation and excellent signal integrity through gigahertz frequencies are required. Additionally, IC power densities can be very high. Microwave circuits require high frequency electrical isolation between circuit components and between the circuit itself and the "outside" world (i.e., off the microwave circuit). Traditionally, this isolation was provided building the circuit on a substrate, placing the circuit inside a metal cavity, and then covering the metal cavity with a metal plate. The metal cavity itself is typically formed by machining metal plates and connecting multiple plates together with solder or an epoxy. The plates can also be cast, which is a cheaper alternative to machined plates. However, one sacrifices accuracy with casting.
One problem attendant with the more traditional method of building microwave circuits is that the method of sealing the metal cover to the cavity uses conductive epoxy. While the epoxy provides a good seal, it comes with a price-high resistance, which increases the loss of resonant cavities and leakage in shielded cavities. Another problem with the traditional method is the fact that significant assembly time is required, thereby increasing manufacture costs.
Moving the microwave signals from point A to point B on the microwave circuit is generally accomplished with transmission lines and waveguides. Transmission lines include coaxial, coplanar and microstrip transmission lines. Waveguides, on the other hand, are typically hollow and offer better performance than transmission lines. Another component often found on a microwave circuit is the resonant cavity. Resonant cavities are used to build microwave filters. These cavities are formed when a dielectric region is completely surrounded by conductive walls.
Waveguide structures and resonant cavities are also formed via machining or casting metal parts. As would be expected, this adds to the cost of the final product.
Another traditional approach to packaging RF/microwave microcircuits has been to attach GaAs or bipolar integrated circuits and passive components to thin film circuits. These circuits are then packaged in the metal cavities discussed above. Direct current feedthrough connectors and RF connectors are then used to connect the module to the outside world.
One method for fabricating an improved RF microwave circuit is to employ a single-layer thick film technology in place of the thin film circuits. While some costs are slightly reduced, the overall costs remain high due to the metallic enclosure and its connectors. Also, dielectric materials typically employed (e.g., pastes or tapes) in this type of configuration are electrically lossy, especially at gigahertz frequencies. The dielectric constant is poorly controlled at both any specific frequency and as a function of frequency. Also, controlling the thickness of the dielectric material often proves difficult.
Another method for fabricating an improved RF microwave circuit is described in Imbedded Waveguide Structures for a Microwave Circuit Package, Ser. No. 08/882460, filed on Jun. 5, 1997, by Ron Barnett et al., which is incorporated by reference herein for all that it teaches. Barnett teaches a method for fabricating imbedded low-loss waveguide structures in microwave packages via an indented cavity formed in the bottom plane of a metal cover plate. The bottom plane of the cover plate is then fused to a metal base plate. An imbedded shielded cavity is formed when the cover plate and the base plate are joined.